1. Field of the Invention
The present invention relates to an improvement of voltage resonance type switching power source apparatus, and especially to an improved power source apparatus capable of operating stably irrespective of the disturbance of the resonance condition caused by an unstable load coupled therewith or some other reasons.
2. Description of the Related Art
As is well known, a switching power source apparatus of a voltage resonance type has a step-up transformer with a primary winding to which an input voltage is applied and a secondary winding by which a high output voltage is supplied for a load. A resonance capacitor is coupled in series with the primary winding of the transformer to form a series resonance circuit, to which a direct current (DC) voltage is applied. Across the capacitor there is provided a switching element such as a transistor, which has a damper diode connected in reverse-parallel therewith.
When the transistor is made to repeat the on-off operation, an alternating voltage is applied to the primary winding of the transformer by the series resonance circuit, so that a high voltage is induced across the secondary winding in accordance with the turn ratio of the transformer. Since the transistor is switched on and off at frequencies as high as 30 KHz, the transformer can be made small in size. Further, if the transistor is switched on and off in accordance with proper timing, the switching loss in the transistor can be decreased. As a result, a small size switching power source apparatus of small size can be realized.
In a voltage resonance type switching power source apparatus as described above, therefore, a signal for switching on a transistor must be produced timely. Otherwise, a large current, which is almost close to a short-circuit current, flows through the transistor, with the result that the transistor fails.
To solve the problem, there has been proposed an apparatus as disclosed in U.S. Pat. No. 4,318,164 (issued on Mar. 2, 1982). According thereto, there is detected a current flowing through a damper diode connected in reverse-parallel to a transistor. The transistor is switched on in synchronism with the detection of the damper diode current. Although details will be described later, the voltage appearing across and the current flowing through the transistor never overlap with each other so that the power loss accompanying the switching operation of the transistor can be greatly reduced.
However, in the case where a load, which consumes power unstably, is coupled with a switching power source apparatus of the voltage resonance type, the condition for the resonance may change irregularly, with the result that a current does not always flow through the damper diode. In such a case, a transistor can not be switched, on and hence the power source apparatus becomes unable to continue normal operation.